Shift left: align preflight-learning-path with review findings

## Context

Follow-on from the epic's review-companion shift-left work (#578, #580). Preflight ([`preflight-learning-path`](.cursor/skills/preflight-learning-path/SKILL.md) skill, [`/preflight-learning-path`](.cursor/commands/preflight-learning-path.md)) is meant to catch review-companion findings before a PR is opened, but hasn't been checked against the same list of recurring rules as creation.

## Scope

- Audit which recurring review findings (see #578) the preflight skill misses or under-checks
- Update `preflight-learning-path` (and `/preflight-learning-path`) to check for the same authoring rules enforced in creation (#580): milestone framing by role, false noop detection, section bookends, navmenu-open/page requirements, selector stability, multistep singletons, focus-before-formfill, secrets `doIt: false`, etc.
- Confirm preflight's checklist stays consistent with `review-guide-pr.mdc` as that doc evolves via #582

## Success criteria

- [ ] Preflight checklist covers the same recurring rules identified in #578/#580
- [ ] Preflight and review checklists don't diverge (single source of truth or explicit cross-reference)

## Priority

Lower priority — sequence after #578/#580 land.

## Related

Part of epic #577. Depends on #578, informed by #580.